Ion selectivity of temperature-induced and electric field induced pores in dipalmitoylphosphatidylcholine vesicles

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Abstract
Temperature and electric field alter the permeability of the bilayer membrane in phospholipid vesicles. A study of cation selectivity of these membrane pores is reported for multilamellar liposomes (MLV) and unilamellar large vesicles (ULV, 95 .+-. 5 nm diameter) of dipalmitoylphosphatidylcholine (DPPC). The permeability of ULV to Rb+ was 1.0 .times. 10-6 .mu.m/s at 22.degree. C and increased to 1.1 .times. 10-5 .mu.m/s at the gel to liquid-crystalline transition temperature (Tm) of the bilayer, at 42.degree. C. The permeability of ULV to Rb+ continued to increase beyond the Tm and reached 1.0 .times. 10-4 .mu.m/s at 56.degree. C, a 100-fold increase over the permeability at 22.degree. C. The permeability of ULV to Na+ showed a local maximum of 6.0 .times. 10-6 .mu.m/s at 42.degree. C and decreased at temperatures higher or lower than the Tm. For MLV, the permeability to both Rb+ and Na+ peaked dramatically at the phase transition temperature, 42.degree. C, and subsided at lower and higher temperatures. When ULV were exposed to an electric field, the permeability to Rb+, Na+, and sucrose surged at a field strength of 30 kV/cm; 30 kV/cm can induce a transmembrane potential of 210 mV. In ULV, the electrically perforated lipid bilayer exhibited selectivity for Rb+ over Na+ only at a narrow electric field range, between 31 and 33 kV/cm. For MLV, no well-defined breakdown voltage was recorded. Thus the fluidlike state of the DPPC bilayer is very permeable to Rb+ but not to Na+ and the permeability characteristics of MLV to Na+ and Rb+ are completely different than those of ULV; the latter exhibit Rb+ selectivity either through thermal fluctuations or through electric perforations, but the former do not.